1. Field of the Invention
This invention relates to optical beam test systems, and more specifically to an interferometer having a micromirror for measuring the quality of an optical beam wavefront.
2. Description of the Related Art
Several systems exist for extracting a reference beam from a beam of a source under test. Such reference beams have been found useful in applications such as interferometry. Some systems use a pinhole, i.e. a very small opening, to generate a reference beam from the beam coming from the source to be tested. Generating a reference beam from the original source beam helps to provide a reference beam which has the same wavelength and a constant phase relationship to the original source beam. Frequently, a pinhole is used to provide a reference beam which is relatively free from the effects of aberration present in the source beam. It is well known that when a sufficiently small pinhole is placed in the path of an aberrated beam, a relatively clean beam is produced because most of the energy present due to the aberration is not passed.
In some existing interferometer systems, a beam expander has been used to remove aberration from the beam used as a reference. An existing interferometer system is described in "A Phase Measuring Radial Shear Interferometer for Measuring the Wavefronts of Compact Disc Laser Pickups", B. E. Truax, Proceedings of SPIE--The International Society for Optical Engineering, Vol. 661 (1986), 74 ("Truax"). In the system described therein, the interferometer is placed at the output of a laser beam source. A beamsplitter splits the source beam into a test beam and a reference beam. An aperture is used in conjunction with a beam expander to remove aberration from the beam to be used for reference.
Such aperture/beam expander systems generally accept a collimated beam input and produce a collimated output. In such systems, the beam is passed through an aperture to filter out aberration energy which is proportionally greater away from the center of the beam. The resulting filtered beam, now narrower, is then expanded, in order to restore it to the width of the source beam.
As stated, such systems generally accept a collimated beam input. Since the beam is collimated, its energy is not as concentrated in the center as in a focused beam. In a 10% aperture--10X beam expander system, the aperture has one tenth the diameter of the beam. Thus, the area of the aperture is approximately one hundredth of the area of the original beam. When such a 10% aperture system is used as typically with a collimated beam, substantially all of the energy of the beam does not pass the aperture. This great loss in energy places a lower limit on the power level of the sources which can be tested in such systems.
In the system described in Truax, the test beam and the reference beam returning from the beam expander are recombined in the beamsplitter and guided by a set of lenses and a mirror to form an interference pattern on the pickup of a video camera. The interference pattern is analyzed to generate data representing the departure of the source beam from that which produces an ideal wavefront.
A disadvantage of the system described in Truax is that the interferometer must be reconfigured to a different setup from that used for testing in order to check and correct the alignment and placement of the system elements and the source. For example, in the system described in Truax, a mirror is slid into place to block light from striking the video camera from the normal direction used in test. The beam from the source is focused onto the center of a surface where a pinhole lies. A portion of the focused beam is reflected back from the surface and guided through alignment lenses to the sliding mirror where it is reflected into the video camera.
An LED is used to illuminate the pinhole from the reverse direction as occurs during the test. Elements of the system are then adjusted and aligned while the pinhole is illuminated from the rear so that the backlit image of the pinhole overlays the image of the focused spot produced by the source to be tested. When alignment is completed, the sliding mirror is slid out of the path of the beam, and the interferometer is reconfigured to the test setup.
Changing the test setup during the alignment operation has the disadvantages of increased system complexity and the inability to permit the alignment to be checked while the system is configured for testing.